It is well known to mount the end of a seat belt on a vehicle body via a linear belt retracting mechanism including a carriage movable on a track. The carriage is typically moved in a belt retracting direction by a spring. A lock is provided to selectively prevent belt extension and is preferably a pawl or the like mounted on the carriage and selectively actuatable to engage the track. The lock is operated by a sensing means responsive to a condition of rapid belt unwinding or a condition of vehicle acceleration.